It is known to provide trenched gates for turning on and off switching devices. U.S. Pat. No. 7,705,368, incorporated herein by reference, describes a vertical insulated gate turn-off (IGTO) thyristor having a vertical NPNP semiconductor layered structure. This forms vertical NPN and PNP bipolar transistors. A cathode electrode is formed on the top n-type layer, and an anode electrode is formed on the bottom p-type layer. The trenched gates extend a certain depth into the upper p-type layer base of the NPN transistor. When a forward bias voltage is applied between the cathode and anode of the device, applying a positive voltage to the gates inverts a portion of the p-type layer base to reduce the base width and increase the beta of the NPN transistor. At this point, the product of the betas of the NPN and PNP transistors is greater than one. The initially small diffusion current is then rapidly amplified by feedback, causing “breakover,” where both the NPN and PNP transistors become fully conducting. To turn off the device, the gate voltage is removed to increase the base width, so the product of the betas is less than one.
Such vertical IGTO devices are more efficient than insulated gate bipolar transistors (IGBTs) and can conduct higher currents due to having a higher current density when on.
One problem with such a vertical IGTO device is that the distance between the bottom of the trenched gate and the bottom of the p-type layer directly affects the turn-on voltage. It is very difficult to precisely control the depths of the trenches, so the turn-on voltage varies from lot to lot. Further, the p-type layer must be fairly deep, and therefore the doping will typically be in-situ doping while the p-type layer is formed. The turn-on voltage is also affected by the doping of the p-type layer. It would be difficult to create a uniformly doped, deep p-type layer using implantation. Further, forming deep trenches for a high voltage device is very time-consuming and therefore expensive. Still further, it is difficult to form other circuitry on the same die as the vertical IGTO device since the IGTO device requires a topside cathode and a bottomside anode.
What is needed is an improvement of the IGTO device that does not suffer from the above-mentioned drawbacks.